The present invention relates generally to wireless communication systems and, more particularly, to mobile station receiver architectures and methods that employ multiple radio receive chains.
As shown in FIG. 1, a wireless communication system 10 comprises elements such as a client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. As illustrated, the communication path from the base station (“BS”) to the client terminal direction is referred to herein as the downlink (“DL”) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (“UL”). In some wireless communication systems the client terminal or mobile station (“MS”) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
Many wireless communication systems use large number of channels over different frequencies. Each base station in these systems use one or more of the available channels. Therefore the client terminals need to make periodic measurements to choose the best channel for communication. The base station to which the client terminal is communicating with is referred as the serving base station. In some wireless communication systems the serving base station is normally referred as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred as a neighbor cell. For these wireless communication systems the client terminals or MSs are required to make periodic measurements on other channels belong to serving or neighbor base stations. These measurements may include parameters such as signal strength. To make such measurements the client terminal may need to tune to other channels in the communication system.
A discussion of the fundamentals of cellular systems may be found in the text entitled “Mobile Cellular Telecommunications Systems” by William C. Y. Lee, copyright 1989 and published by McGraw-Hill Book Company, the entire disclosure of which is hereby expressly incorporated by reference herein. Another text that details aspects of cellular communication systems is “Wireless and Personal Communications Systems” by Garg and Wilkes, copyright 1996 and published by Prentice Hall PTR, the entire disclosure of which is hereby expressly incorporated by reference herein.
As shown in FIG. 2, client terminal/MS 12 typically includes a baseband subsystem 16 and a radio frequency (“RF”) subsystem 18. Memory 20, such as an external memory, is shown connected to the baseband subsystem 16. The baseband subsystem 16 normally consists of a micro controller unit (“MCU”) 22, a signal processing unit (“SPU”) 24, data converters 26, peripherals 28, power management 30, and memory 32 as shown in FIG. 3. The SPU 24 may be a digital signal processor (“DSP”), hardware (“SPU”) accelerators, co-processors or a combination of the above. Normally the overall control of the baseband subsystem 16 is performed by software running on the MCU 22 and the processing of signals is done by the SPU 24.
Analog to digital converters (“ADCs”) convert a received analog signal into digital for the baseband system to process it. Similarly, digital to analog converters (“DACs”) convert the processed baseband digital signal into analog for transmission. The ADCs and DACs are collectively referred to herein as “data converters” 26. The data converters 26 can either be part of the baseband subsystem 16 or the RF subsystem 18. Depending on the location of the data converters 26, the interface between the two subsystems will be different. The location of the data converters 26 does not alter the overall function of the client terminal.
An RF subsystem 18 normally consists of a receiver, a transmitter, a synthesizer, a power amplifier, an antenna, and other components. An RF subsystem 18 for a time division duplex (“TDD”) system is shown in FIG. 4. Receiver section 34 performs the task of converting the signal from RF to baseband. It includes mixers 36, filters 38, low noise amplifiers (“LNAs”) 40 and variable gain amplifiers (“VGAs”) 42. Transmitter section 44 performs the task of converting the baseband signal up to the RF. It includes mixers 46, filters 48, and gain control through VGAs 50. Power amplification of the transmit signal is typically done by a separate power amplifier (“PA”) unit 52 but is considered part of the transmit RF chain. In some architectures, some of the components of the receiver and transmitter can be shared. As shown, the receiver section 34 and the transmitter section 44 are coupled to an antenna 54 via a transmit/receive switch 56. Synthesizer 58 is also shown as coupling to the receiver section 34 and the transmitter section 44.
Down conversion in the receiver 34 and up conversion in the transmitter 44 can be done in a single stage or multiple stages which lead to different implementations of RF subsystems. One possible implementation is direction conversion or zero intermediate frequency (“IF”) where the downlink RF signal is converted to baseband by a single mixer and local oscillator (“LO”). Another implementation employs a super-heterodyne structure which uses one or more IF stages and LOs during the process of converting the RF signal to baseband. Yet another implementation uses an approach called “low IF” that converts the analog RF signal to a low intermediate frequency and then convert the analog intermediate frequency to a digital signal using high speed data converters.
The synthesizer 58 produces the LO frequency needed by the receiver 34 and the transmitter 44 to convert the signal from RF to baseband and to convert from baseband to RF respectively. A frequency synthesizer is an electronic system for generating a range of frequencies from a single fixed reference frequency. Synthesizer 58 normally consists of a phase-frequency discriminator, a charge pump, a loop filter, a voltage controlled oscillator (“VCO”) and a frequency divider. One input to the synthesizer is a reference clock frequency and the other input is the desired frequency at the output of the VCO. The desired frequency is converted into an appropriate value for the frequency divider such that the VCO produces the desired frequency.
Typically the process of generating the LO frequency for receiving from or transmitting on a particular RF channel is referred to as “tuning to a channel.” The mixers take the LO frequency generated by the synthesizer and multiply the desired signal. The output of the mixer can be filtered appropriately depending on whether down conversion (in receivers) or up conversion (transmitter) is desired.
Often, multiple receive and transmit chains are used in wireless communication systems to improve performance. The performance improvement can be in terms of better coverage, higher data rates, multiplexing of multiple users on the same channel at the same time, or some combination of the above. FIG. 5 illustrates an RF subsystem 60 with two RF receive chains.
As shown, RF subsystem 60 includes a transmitter 62, a synthesizer 64, and a pair of receivers 661 and 662. One of the receivers, 661, and the transmitter 62 are coupled to a first antenna 681 via transmit/receive switch 70. The other receiver, 662, is connected to a second antenna 682. Different techniques using multiple receive and/or transmit chains are often referred to with different names such as diversity combining (maximum ratio combining, equal gain combining, selection combining, etc.), space-time coding or space-time block coding, and multiple input multiple output (“MIMO”).
In a traditional receiver with multiple chains, whenever the multiple receive chains are used they are all tuned to the same channel.
For instance, conventional multiple receive chain systems may employ multiple antennae and multiple RF chains as shown in FIG. 6. As shown, system 76 includes multiple receive chains 781, 782, . . . , 78N. Each receive chain 78 is coupled to a respective antenna 801, 802, . . . , 80N. Synthesizer 82, which is fed by a reference oscillator 83, couples to each of the receive chains 78 and to baseband processor 84. The synthesizer 82 provides a local oscillator signal LO to the receive chains 78. And the baseband processor 84 includes respective in-phase and quadrature (“I/Q”) ADCs 861, 862, . . . , 86N that couple to respective ones of the receive chains 78.
In multiple receive chain systems, typically the antennae are designed to cover the entire frequency band of operation. However, the RF signal chains are tuned to a particular channel in the frequency band of operation. Commonly in a multiple receive chain MIMO configuration all RF signal chains are tuned to the same exact synthesizer frequency. The signals in all RF chains are different because of different positions of the various antennae 801, 802, . . . , 80N.
In mobile communication systems, the MS typically communicates with one cell, normally referred to as serving cell. To facilitate mobility while maintaining continuous link with the serving cell in the network, the MS must periodically find, receive, update, and manage information about neighbor cells by performing signal measurements on them.
In order to obtain neighbor cell reception in some situations, the MS either has to switch between serving cell and neighbor cell or have additional dedicated RF signal chains that can be tuned to a different channel for neighbor cell reception as shown in FIG. 7. As shown here in system 76′, an additional RF receive chain 88 having an antenna 89 is employed for a neighbor cell. Receive chain 88 couples to a respective I/Q ADC 90 in the baseband processor 84. Furthermore, an additional synthesizer 92 receives input from reference oscillator 94 and the baseband processor 84 and provides a LO signal to the RF receive chain 88.
Unfortunately, existing wireless systems implementing multiple receive chains have various drawbacks and disadvantages. For instance, switching between a serving cell and a neighbor cell may result in performance degradation. And employing additional dedicated RF chains for neighbor cell reception leads to additional cost and space requirements in the MS. Thus, there is a need for improved architectures that efficiently employ multiple receive chains.